Presently there are a number of optic interferometry techniques and instruments for measuring geometric sizes, such as distance, surface form, dimensions, movement and vibration
Norwegian patent 314323 and its patent family is one example which describes hardware and algorithms to measure vibrations in small structures using an interferometer and using so-called time average recordings. This is a two-dimensional solution with a single sensitivity direction for the measurement.
Three-dimensional vibration recordings and systems for three-dimensional measurements are known in prior art.
A main problem with three-dimensional measurements is to measure and find the vibration phase relation between different directions. If the object vibrates with a steady state vibration with constant frequency and amplitude, it is possible to measure vibrations in one direction at a time, for instance in the X-direction first, then in the Y-direction and finally in the Z-direction. But to find the phase relation between the different directions and different measurements may be difficult.
One way to solve this is by performing measurements in the different directions simultaneous at the same time, and to get the temporal relation between different measurements.
If the object under investigation is imaged from a specific direction, most interferometric systems measure vibrations in this observation direction, for instance as described in the applicant's Norwegian patent 314323. The illumination of the object, done with a laser beam or with another (partly) coherent light source, will then be in-line with the observation direction. For in-plane measurements, where the sensitivity direction is perpendicular, or at least with a component perpendicular, to the observation direction, the illumination normally comes from another direction than the observation direction, when interferometric methods are used. A typical prior art setup for in-plane measurements with a sensitivity direction perpendicular to the observation direction, consist of two illumination light beams from each side, as shown in FIG. 1. In this case, no internal reference beam is required in the interferometer, as interference and interferometric sensitivity is achieved by interference between the two illumination light beams.
With the setup in FIG. 1, measurements can be done in one in-plane direction. The normal procedure will then be to illuminate the object by two other light sources from other directions, to achieve sensitivity in the other in-plane direction. The two first light sources can also be moved to new positions for this second recording. A disadvantage with this way to perform in-plane measurements in two directions is the need for space for illumination sources in different directions around the object under investigation.
Prior art solutions further suffer from low free space available at one side of the object to be investigated and difficult access to the object for the user.
If four illumination light beams are being used, two at a time, or alternatively, all four at the same time, for in-plane measurements in two directions, it may also be complicated to find the phase relation between the measurements in the two in-plane directions.
There are also some systems for in-plane measurement based on principles with stroboscopic illumination or stroboscopic imaging. These systems have problems with the measurement of high frequencies and also problems with limited amplitude resolution.
There is accordingly a need for a method and interferometer which can be used for both two-dimensional recordings and full three-dimensional recordings of vibrations in small and microscopic objects.
There is accordingly a need for a method and interferometer which are capable of performing both in-plane measurements and out-of-plane measurements.
The disclosed embodiments provide a method and an interferometer which solves the above mentioned problems with prior art.
The disclosed embodiments also provide a method and interferometer arranged for providing measuring of vibrations in two or three directions in objects.
Also provided is a method and interferometer providing vibration phase relation between the two in-plane directions.
Also provided is a method and interferometer also providing vibration phase relation between the two in-plane directions and a third out-of-plane direction.
Also provided is a method and an interferometer also providing free space at one side if the investigated object to provide easier access to the object for the user.
Also provided is a method and an interferometer for measuring an object with two or more sensitivity directions, one at a time, where the object is illuminated by one or more coherent illumination light beams at a time.
Also provided is a method and an interferometer capable of performing both in-plane measurements and out-of-plane measurements.
Also provided is a method and an interferometer which use three illumination light beams for in-plane measurements, where one of the three illumination light beams is common for in-plane measurements in two directions.
Also provided is a method and an interferometer arranged for using two illumination light beams for measurement in an out-of-plane direction, and where one of the two illumination light beams is used as a reference beam.
Also provided is a method and an interferometer arranged for phase modulation of illumination light or reference beam for out-of-plane measurements.
Also provided is a method and an interferometer arranged for translation and/or rotation of diffusors in illumination light beam paths to provide changes in speckle patterns.
Also provided is a method and an interferometer providing full three-dimensional recordings of vibrations in especially small and microscopic object, which also is applicable on larger structures.
The disclosure also concerns using phase modulation and time average recordings to be able to measure at high frequencies in all directions (in-plane and out-of-plane).
The disclosure provides a method and an interferometer for measuring vibration in two or three directions for an object under investigation.
More particularly, provided herein is a method and an interferometer which involves the use of an imaging system in an interferometric configuration, where an object is illuminated from different directions in turn, to obtain measurements with sensitivity in three directions, preferably X-, Y- and Z-directions in a coordinate system.
This will provide full three-dimensional vectorial vibration displacement of the object under investigation.
According to the disclosure the object can be imaged onto a detector array, which e.g. can be a ccd array.
The object is vibrating at one frequency at a time, and the object vibration excitation is arranged to be controllable by a control unit arranged to the interferometer.
According to the disclosure the object is vibrating over a certain period of time with constant frequency and constant amplitude. The measurement takes place over this period of time, which can last from less than a second and up to several minutes.
So-called time average recording is utilized, where a detector array exposes over one or many vibration periods, but the disclosed method and device can also be used with recording principles involving short exposure time compared to the vibration period. The object is measured with two or more sensitivity directions, one at a time, where the object is illuminated by one or more coherent illumination light beams at a time. Within a XYZ coordinate system, measurements are preferably performed in the X- and Y- and Z-direction, in turn. If the XY-plane represents the in-plane directions for the object, and the Z-direction represents the out-of-plane direction, then the Z-measurement is performed by illuminating the object with a single illumination light beam and also by the use of a reference beam in addition, which is expanded to illuminate the detector array directly.
For in-plane measurements, two illumination light beams are used for the X-direction, and two illumination light beams are used for the Y-direction, where one illumination light beam is common for the two directions.
The disclosed interferometer preferably includes a modulator and a control unit. The control unit controls the excitation of the object and also controls a reference modulator at the same time. The modulator works like a temporal reference for signals applied for object excitation and measurement in all directions X, Y and Z.
Accordingly, described herein is a method and interferometer which can be used for both two-dimensional recordings and full three-dimensional recordings of vibrations in small and microscopic objects. They can also be used on larger structures.
A method of measuring vibrations in two or three dimensions in objects accordingly includes using three illumination light beams for measurement of in-plane vibrations in two directions, where one of the three illumination light beams is common for the two in-plane measurements, and where the three illumination light beams are configured to provide free space at one side of the object for easier access to the object for the user.
The method can further use two illumination light beams for measuring vibrations in the object in a third direction, which third direction is out-of-plane direction, where one illumination light beam is illuminating the object through an imaging objective and one illumination light beam is used as reference beam.
The method can further include using an algorithm where the object is excited with one frequency at a time, and where one or more phase modulators are used to provide a dynamic phase modulation of one of the two illumination light beams being used for each recording in each direction.
The method can further include using the same modulator for the two in-plane measurements as one of the illumination light beams are phase modulated and this illumination light beam is common for the two in-plane measurements.
The method can further include using the same, or a different phase modulator for the measurement in the third out-of-plane direction, where the modulator also phase modulates one of the two illumination light beams used for out-of-plane measurements.
The method can further include using optical diffusors in the illumination light beams for in-plane measurements, and moving the diffusors by translations and/or rotations to obtain changes in speckle patterns in the illumination light beams.
The method can further include using an optical diffusor in the object illumination path for the out-of-plane measurement, and moving the diffusor by translations and/or rotations to obtain changes in speckle pattern in the illumination light beam.
The method can further include using a polarizing beam splitter and a retarder plate or quarter plate in the illumination path for the out-of-plane measurement for avoiding that reflections from the objective lens reach the detector array.
The method can further include using a full field detector array for detection of signals in the interferometer.
The method can further include performing multiple recordings, where each recording is performed with different levels of intensity in illumination light beams and in detector array images.
The method can further include using an algorithm where signals used for different points or areas of an imaged area are selected from different recordings with different levels of intensity, as described above, for obtaining good or acceptable signals from all points, or most of the points, on the object surface.
The method can further include performing measurement of vibrations in three, alternatively two, separate directions using the same modulator signal as temporal reference for the three- or two-dimensional measurements.
The method can further include using a steady state object excitation with constant amplitude and frequency for the object during the recording in three or two directions.
The method can further include combining the three or two dimensional measurements to achieve a full three or two dimensional displacement vector for all points on the imaged surface for object frequency and amplitude used in the three or two dimensional measurements.
The method can further include plotting a full three- or two-dimensional animation of the object surface displacement.